Heat shrinkable materials today enjoy widespread industrial usage. These articles are characterized by their ability to shrink upon being heated. Comprised of polymers, they are first fabricated in their desired final size and shape and then crosslinked as by the use of high energy radiation or chemical crosslinking techniques. Subsequent heating of the material melts the crystals or significantly lessens other internal molecular forces to an extent sufficient to allow distortion above the melt of the product. Quenching of the heated and distorted crosslinked material results in a product which is in its distorted shape while at room temperature. Reheating of the product sufficiently to melt the crystals or reduce intermolecular forces allows the crystalline material quickly to return the product to its initial crosslinked size and shape. Methods of manufacturing these articles are exemplified in U.S. Pat. Nos. 2,027,962 and 3,086,242.
Heat shrinkable materials have heretofore been used in the telecommunications industry in the form of end caps having a tubular sleeve portion formed unitarily with an end wall portion that is bonded about the ends of cables. For example, it is often desirable to mount an end cap temporarily about the end of a communications cable to prevent loss of internal cable pressurization and internal flooding compound to ambience and to inhibit ambient air and moisture from entering the cable. In this application the end cap sleeve portions are formed of heat shrinkable materials of an expanded size having an inside diameter greater than the outside diameter of the cable whereby they may be easily placed over the end of the cable and then shrunk tightly down upon the cable by the application of heat. Cable end seals effected by these end caps have, however, often proved to be insecure and leaky allowing pressurization to be lost and moisture to enter the cable core over a period of time. Upon examination it has been revealed that the reason for this has typically been an incomplete fusion of the heat shrinkable plastic end cap to the plastic jacket of the cable. In other words, even though the end cap has been shrunk tightly upon the substrate, fusion of the two plastic materials at their interfacing surfaces has been less than that sufficient to produce a sound and hermetically reliable seal.
Hot melt adhesive coatings have also been applied to the surface of the jacket to aid in bonding the end cap to the jacket upon shrinkage. Although this approach has produced more reliable seals they are achievable only at a substantial increase in costs. Furthermore, in some cases, as where the substrate has a size discontinuity or step, it has proven difficult to maintain such heat shrinkable end caps or sleeves in proper positions during seal formations since the outer member often tends to "walk off" the inner member where an adhesive coating is present.
Thus, it is desirable to achieve a sound hermetic seal between a heat shrinkable end cap or sleeve and substrate without the use of fusible inserts or adhesive coatings and by the very mechanism of heat application used in drawing the end cap or sleeve down upon the substrate. One method of achieving this is disclosed in U.S. Pat. No. 3,691,505 with the use of a laminate tube or sleeve having a non-fusible heat shrinkable outer layer and an inner fusible layer, the outer layer having a relatively higher melting point than the inner layer. By applying hot air the tube may be shrunk down upon a semi-vulcanized butyl rubber tape covering a heater cable and the laminate inner layer fused to the tape. Though this approach does avoid the use of an adhesive per se it remains analogous in that a multi-layered heat shrinkable tube or sleeve is still employed with its attendant costs.
Accordingly, there still remains a need for hermetic seals between heat shrinkable sleeves and plastic substrates which overcome the problems and limitations experienced with those of the prior art. It is to this task which the present invention is primarily directed.